Sigam 105mm macro OS issue

I have just purchased a brand new sigma 105mm f/2.8 ex dg os hsm macro from Jessops for my Nikon D7100, the image quality produced with the lens is great, the only problem I have is with the OS switched on when focusing on a subject no matter how steady the camera is held the image in the viewfinder moves from the point of focus on its own, first I thought it was me not holding the camera steady but the same thing happened when locked perfectly still mounted on my tripod, the problem does not occur with the OS switched off.

Just want to know if anyone else has had the same problem with this lens, is this a fault or just a common problem with the sigma 105mm f/2.8 ex dg os hsm macro.